I have been told (not sure if it is true), that mirror (and glass) do not allow to pass the electromagnetic signals of mobile signals.
but for a standard mirror what type of wave can pass through it?
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The answer I provided to this older question is very applicable here. The degree to which an EM signal is obstructed will depend on the material properties of the object (conductivity, primarily) and the wavelength of the signal. For a mobile phone, the conductive metal coating of the mirror is likely much thinner than the skin depth, so the signal should only be weakened a little bit.